1. Field of the Invention
The present invention relates to a front end interface circuit for a communications network, and more particularly, to an automatic impedance matching circuit for separating upstream and downstream signals in the network.
2. Background
The telecommunications industry has developed analog receiver front end circuits that are used for separating the flows of upstream and downstream signals in digital two-way communications networks, and more particularly, to SDV networks. The analog front end circuit, also called an interface circuit, is typically a three-port device that has a downstream input, a downstream output which also serves as an upstream input, and an upstream output that is different from the downstream input. Conventional interface circuits typically include diplexers, active hybrids and passive hybrids. These interface circuits are implemented in optical network units (ONU) in fiber-to-the-curb (FTTC) architectures, described in Harman, et al., "Local Distribution for IMTV," IEEE Multimedia, Vol. 2, No. 3, IEEE Computer Society, Fall 1995, incorporated herein by reference.
Both active and passive hybrid circuits require resistor value tuning in the radio frequency (RF) signal path in order to match the impedance of the downstream input transmission line to that of the output downstream transmission line, which is typically an unshielded twisted pair (UTP). The output downstream transmission line is connected to a receiver, which is typically at a customer's site that could be thousands of feet away from the analog front end interface circuit. A typical receiver at the downstream output includes a Lucent Technologies T7665 quadrature phase shift keying (QPSK) receiver, which is implemented on a monolithic integrated circuit (IC). The T7665 IC is able to receive downstream signals from either a twisted pair or a coaxial transmission line, although a UTP transmission line is typically used. Without resistor value tuning in the downstream RF signal path in the active hybrid or passive hybrid circuit to match the UTP line impedance, the specification for out-of-band signal attenuation, which is typically on the order of 75 dB, may not be met. Although manual tuning of resistors is acceptable for experimental or non-production circuits, it is generally not acceptable for production units due to expensive and cumbersome tuning processes. Therefore, neither the active hybrid circuits nor the passive hybrid circuits are feasible for commercially competitive large-scale production. Therefore, there is a need for a front end interface circuit that obviates the necessity of manual resistor value tuning.
A diplexer is basically a three-port circuit with an input port and two output ports, each output port connected to a bandpass filter with a different passband. A diplexer circuit does not require resistor value tuning. However, since an analog front end circuit for a digital interactive network usually has stringent out-of-band signal attenuation specifications, which are typically on the order of 75 dB, the diplexer requires complicated bandpass filter designs. Therefore, there is a further need for a front end interface circuit without stringent specifications for bandpass filters.